Drugs that have abuse liability in humans typically serve as positive reinforcers to maintain and strengthen behavior leading to their administration in animals and serve as discriminative stimuli controlling two-lever choice behavior. Experiments are being conducted to assess neuropharmacological and behavioral mechanisms underlying drug self-administration behavior and behavior controlled by drugs as discriminative stimuli in rats and monkeys and the ability of pharmacological or behavioral manipulations to modify such behavior. We previously demonstrated persistent intravenous self-administration behavior by experimentally and drug-naive squirrel monkeys for doses of THC comparable to those in marijuana smoke inhaled by humans (Justinova et al. 2003), which showed that this drug possesses reinforcing properties of its own that are not dependent on prior self-administration of other drugs (such as cocaine; see Tanda et al. 2000). Thus, self-administration of THC by squirrel monkeys provides a reliable animal model of human marijuana abuse, suitable for studying the relative abuse liability of other natural and synthetic cannabinoids. We have now used this animal model to study anandamide, an endogenous ligand for brain cannabinoid CB1 receptors, produces many behavioral effects similar to those of THC, the main psychoactive ingredient in marijuana. There is only indirect evidence that endogenous cannabinoids such as anandamide participate in brain reward processes. We have now shown that anandamide serves as an effective reinforcer of drug-taking behavior when self-administered intravenously by squirrel monkeys (Justinova et al. 2005). We also showed that methanandamide, a synthetic long-lasting anandamide analog, similarly serves as a reinforcer of drug-taking behavior. Finally, the reinforcing effects of both anandamide and methanandamide were blocked by pretreatment with the cannabinoid CB1 receptor antagonist rimonabant (SR141716). These findings strongly suggest that release of endogenous cannabinoids is involved in brain reward processes and that activation of cannabinoid CB1 receptors by anandamide could be part of the signaling of natural rewarding events. There is increasing experimental evidence from animal studies for reciprocal functional interactions between endogenous brain cannabinoid and opioid systems. Most of the evidence for a role of opioid systems in the modulation of the reinforcing effects of cannabinoids is indirect and comes from behavioral studies in rodents. We tested the hypothesis that the opioid system may play a role in the addictive properties of cannabinoids in non-human primates (Justinova et al. 2004). We used naltrexone, an opioid antagonist clinically used for the treatment of opiate abuse or alcoholism. Pretreatment with naltrexone in squirrel monkeys self-administering THC resulted in a significant reduction of the self-administration behavior maintained by THC. In contrast, naltrexone pretreatment had no significant effect on cocaine self-administration responding under identical conditions. This experiment indicates that blockade of opioid receptors can modulate the addictive effects of THC in non-human primates. Accumulating evidence suggests that the endogenous cannabinoid system is involved in the reinforcing effects of heroin. In rats intravenously self-administering heroin, we investigated effects of cannabinoid CB1 receptor agonists and compounds that block transport or metabolism of the endogenous cannabinoid anandamide (Solinas et al., 2005). The natural cannabinoid CB1 receptor agonist THC did not alter self-administration of heroin under a fixed-ratio one schedule. Under a progressive-ratio schedule, however, THC dose-dependently increased the number of heroin injections self-administered per session and the maximal ratio completed (break-point). The synthetic cannabinoid CB1 receptor agonist WIN55,212-2 had effects similar to THC under the progressive-ratio schedule. In contrast, AM404, an inhibitor of transport of anandamide, and URB597, an inhibitor of the enzyme fatty acid amide hydrolase that degrades anandamide, or their combination, did not increase reinforcing efficacy of heroin at any dose tested. Thus, activation of cannabinoid CB1 receptors facilitates the reinforcing efficacy of heroin and this appears to be mediated by interactions between cannabinoid CB1 receptors and mu-opioid receptors and their signaling pathways, rather than by an opioid-induced release of endogenous cannabinoids. Although cannabis possesses addictive properties, there is a continuing debate on the possibility of legalizing cannabis use for medical purposes and decriminalizing its recreational use. Gateway drug hypothesis says that cannabis use could increase the probability of encountering, trying and eventually becoming addicted to other illicit drugs such as heroin. In a recent study (Solinas et al. 2004), we investigated the effects of pre-exposure to THC on subsequent intravenous self-administration of heroin by Sprague-Dawley rats. We used a level of exposure previously shown to produce behavioral sensitization to THC and cross-sensitization to morphine. Rats pre-exposed to THC subsequently self-administered significantly more heroin injections per session and showed significantly shorter post-injection pauses over a range of heroin doses using the variable-dose schedule. Interestingly, the maximum effort rats would exert to receive an injection of the different doses of heroin under the progressive-ratio schedule was not altered by THC pre-exposure. Our results demonstrate that pre-exposure to THC alters some pharmacological effects of heroin that determine frequency of heroin-taking, but offers no support for the hypothesis that pre-exposure to THC alters heroin?s efficacy as a reinforcer. We also have conducted a series of experiments to better characterize the pharmacological effects of ligands that may be useful to treat nicotine dependence. First, we have evaluated the effects of rimonabant, an antagonist at cannabinoid CB1 receptors, in rats trained to discriminate nicotine from saline under a fixed-ratio schedule of food delivery. In contrast to nicotine replacement therapy and bupropion, rimonabant did not have nicotine-like discriminative effects and did not alter the dose-response curve for nicotine discrimination. Other experiments have revealed that rimonabant blocks nicotine-induced place preference. Therefore, these findings support the proposed use of rimonabant for smoking cessation and indicate that it would selectively reduce the influence of environmental stimuli that contribute to persistent smoking behavior, without affecting subjective responses to nicotine. A similar profile has been found with selective dopamine D3R ligands. We have reviewed the utility of these ligands for the treatment of drug dependence. In another set of experiments, we have evaluated the effects of ethanol in rats trained to discriminate nicotine from saline under a fixed-ratio 10 schedule of food delivery. Ethanol did not produce any nicotine-like discriminative effects and did not produce any shift in the dose-response curve for nicotine discrimination. Thus, the ability to discriminate nicotine's effects does not appear to be altered by ethanol administration. However, the high dose of 1 g/kg ethanol, given either alone or in combination with nicotine, markedly depressed food-maintained responding. This later effect was associated in some rats with an attenuation of the discriminative-stimulus effects of the training dose of nicotine. This suggests that past reports of increased tobacco smoking following ethanol consumption in humans are connected, in some way, with an increase in motivation to consume nicotine that is produced by ethanol, rather than with a decrease in the subjective response to nicotine.